Comparison of Broadband Decoupling Schemes

Many NMR measurements such as HSQC or HMQC rely on broadband X nucleus decoupling (X = ¹³C, ¹⁵N, ³¹P .... etc.)..  Broadband decoupling schemes, using conventional rectangular pulses (e.g GARP) require fairly high power levels leading to undesired sample heating.  They are also limited in their effective decoupling bandwidth.  Adiabatic decoupling schemes use shaped adiabatic pulses and have become more and more common over the last couple of decades due, in large part, to the flexibility of modern NMR instruments to generate shaped pulses.  Adiabatic decoupling schemes (e.g. WURST) use much less power than those using conventional rectangular pulses. thereby reducing or eliminating problems associated with sample heating.  Due to the lower power requirements and increased effectiveness over wider frequency ranges, adiabatic decoupling schemes are ideally suited for X nucleus decoupling at higher field strengths. The figure below shows 500 MHz ¹H [³¹P] NMR spectra measured with inverse gated decoupling for the P-CH₃ methyl resonance of dimethyl methylphosphonate  The single scan spectra were collected in a pseudo-2D fashion, as a function of the decoupler offset frequency from -256 ppm to +256 ppm from the ³¹P resonance frequency in 1 ppm steps.  The acquisition time and recycle time for each FID were 2 sec and 4 sec, respectively.   In the left-hand panel, broadband GARP decoupling was employed at a power of 1.22 W  (60 µsec 90° pulses).  In the right-hand panel, WURST decoupling was used at a peak power of 0.755 W (2 ms WURST pulses, bandwidth = 250 ppm).

Clearly, the data collected with WURST decoupling, at lower power, have a much larger decoupling range (250 ppm) compared to the data collected with GARP decoupling (98 ppm).  Furthermore, while the GARP data were collected, the sample temperature increased and had to be compensated for by the variable temperature unit.  No such temperature increase was observed while collecting  the WURST data.  It is also interesting to note that, in the case of GARP decoupling, distorted line shapes are observed just outside of the decoupling range, while for WURST decoupling, the spectra are fully coupled just outside of the decoupling range with a very sharp transition between being fully coupled and fully decoupled.  For broadband decoupling, WURST is best!   

The limitations of 19F GARP Decoupling

In a previous post, it was shown that distorted line shapes are obtained for resonances in broadband decoupled NMR spectra when the resonances of the decoupled nuclide are outside of the effective decoupling bandwidth.  This can be a particularly difficult problem when observing ¹H NMR spectra with ¹⁹F decoupling.  ¹⁹F has a large chemicals shift range so, if there are multiple widely spaced ¹⁹F resonances, it will be difficult or impossible to decouple all ¹⁹F sites at once, particularly at higher magnetic field strengths.  If one is not aware of this problem, data misinterpretation may be an issue as distorted line shapes will  lead incorrect splittings used to measure coupling constants.  The figure below illustrates this problem.  The top three panels of the figure show the 300 MHz ¹H[¹⁹F] NMR spectra for the three ¹H resonances of 1,2-difluoropyridine as a function of the ¹⁹F decoupler offset.  The GARP decoupling scheme was used with 90° pulses of 80 µsec.  The decoupler offsets, depicted in the bottom panel of the figure, were varied in 5 ppm increments.

Of the 11 decoupler offsets used, only offset 6 (at -116 ppm) effectively decoupled both ¹⁹F sites.  Varying the decoupler offset by only ± 5 ppm leads to distorted line shapes, which are particularly pronounced for the H₃ resonance.  These distorted line shapes could easily lead to data misinterpretation and erroneous coupling constants.  In this case, the ¹⁹F decoupling bandwidth is 55 ppm.  Since the chemical shift difference between the two  ¹⁹F resonances is 52 ppm, one is able to obtain a fully ¹⁹F decoupled ¹H spectrum with the careful choice of the decoupler offset frequency however, there will be cases where the decoupling bandwidth would not be sufficient to decouple all ¹⁹F resonances in some molecules.  How then can one generally evaluate all of the coupling constants in fluorine containing molecules?  The ¹⁹F-¹⁹F couplings can be evaluated in a ¹⁹F[¹H] spectrum (not shown).  Specific ¹H-¹⁹F coupling constants can be determined by measuring a ¹H PSYCHE spectrum or be collecting ¹H spectra with selective ¹⁹F continuous wave (CW) decoupling  for each of the¹⁹F resonances.  The latter is shown in the figure below.  The bottom panel shows a standard ¹H spectrum.  The middle two panels show the ¹H spectra for each of the ¹⁹F sites decoupled separately using CW decoupling.  The top panel shows the fully ¹⁹F decoupled spectrum.

Using these data, all of the coupling constants can be evaluated and are shown in the figure below.

In conclusion, one must be careful in interpreting ¹H[¹⁹F] spectra and understand the limits of the ¹⁹F decoupling scheme used.

Decoupling Bandwidth and Distorted Line Shapes

Broadband X nucleus decoupling (X = ¹³C, ¹⁵N, ³¹P, ¹¹B, ¹⁹F etc.....) is frequently used in ¹H detected 2D HSQC/HMQC data collection or in standard 1D ¹H spectra to aid in structure assignment.  When broadband decoupling schemes are used, one must keep in mind that they are not infinitely broadbanded.   They have finite bandwidths over which they are effective thus limiting the chemical shift range for the decoupled nuclide.  The effective bandwidth depends on the particular decoupling scheme and the decoupling power used.  If multiple peaks are to be decoupled, one must insure that all peaks are within the decoupler bandwidth.  One can determine experimentally the effective decoupling bandwidth by running a series of ¹H spectra varying the decoupler offset frequency.  Such a measurement is shown in the figure below for the P-CH₃ methyl resonance of dimethyl methylphosphonate. 300 MHz ¹H [³¹P] NMR spectra were collected in a pseudo-2D fashion, incrementing the decoupler offset frequency from 200 ppm to -200 ppm from the ³¹P resonance frequency in 1 ppm steps.  Broadband GARP decoupling was employed with a power of 3125 Hz  (80 µsec 90° pulses).  The pseudo-2D contour plot is shown in the left-hand panel and a stacked plot is shown in the right-hand panel.

One can see that the effective decoupling bandwidth is 15.56 kHz or 128 ppm on a 300 MHz instrument.  When the decoupler offset exceeds ±64 ppm from the ³¹P resonance frequency, one obtains distorted line shapes.  Representative distorted line shapes are shown in the figure below.  The bottom spectrum was collected with no ³¹P decoupling.  The top, fully decoupled spectrum was collected with on-resonance ³¹P GARP decoupling.  The middle spectra, highlighted in pink, are representative distorted spectra outside of the effective decoupling bandwidth.

If one is not aware of the decoupling offset and available bandwidth, one may obtain misleading line shapes subject to misinterpretation.